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Integrins, and integrin-mediated adhesions, have long been recognized to provide the main molecular link attaching cells to the extracellular matrix (ECM) and to serve as bidirectional hubs transmitting signals between cells and their environment. Recent evidence has shown that their combined biochemical and mechanical properties also allow integrins to sense, respond to and interact with ECM of differing properties with exquisite specificity. Here, we review this work first by providing an overview of how integrin function is regulated from both a biochemical and a mechanical perspective, affecting integrin cell-surface availability, binding properties, activation or clustering. Then, we address how this biomechanical regulation allows integrins to respond to different ECM physicochemical properties and signals, such as rigidity, composition and spatial distribution. Finally, we discuss the importance of this sensing for major cell functions by taking cell migration and cancer as examples.Integrin extracellular matrix receptors establish contacts between the cell interior and the cell microenvironment. Integrins are subjected to complex biochemical and mechanical regulation, which allows cells to respond to extracellular matrix with different physicochemical properties and fine-tunes cell behaviour.

Eukaryotic cells can undergo different forms of programmed cell death, many of which culminate in plasma membrane rupture as the defining terminal event 1 – 7 . Plasma membrane rupture was long thought to be driven by osmotic pressure, but it has recently been shown to be in many cases an active process, mediated by the protein ninjurin-1 8 (NINJ1). Here we resolve the structure of NINJ1 and the mechanism by which it ruptures membranes. Super-resolution microscopy reveals that NINJ1 clusters into structurally diverse assemblies in the membranes of dying cells, in particular large, filamentous assemblies with branched morphology. A cryo-electron microscopy structure of NINJ1 filaments shows a tightly packed fence-like array of transmembrane α-helices. Filament directionality and stability is defined by two amphipathic α-helices that interlink adjacent filament subunits. The NINJ1 filament features a hydrophilic side and a hydrophobic side, and molecular dynamics simulations show that it can stably cap membrane edges. The function of the resulting supramolecular arrangement was validated by site-directed mutagenesis. Our data thus suggest that, during lytic cell death, the extracellular α-helices of NINJ1 insert into the plasma membrane to polymerize NINJ1 monomers into amphipathic filaments that rupture the plasma membrane. The membrane protein NINJ1 is therefore an interactive component of the eukaryotic cell membrane that functions as an in-built breaking point in response to activation of cell death. Structural, biochemical and mutagenesis studies indicate that, in dying cells, the membrane protein NINJ1 assembles into filaments, disrupting the cell membrane.

Ferroptosis is a type of cell death that was described less than a decade ago. It is caused by the excess of free intracellular iron that leads to lipid (hydro) peroxidation. Iron is essential as a redox metal in several physiological functions. The brain is one of the organs known to be affected by iron homeostatic balance disruption. Since the 1960s, increased concentration of iron in the central nervous system has been associated with oxidative stress, oxidation of proteins and lipids, and cell death. Here, we review the main mechanisms involved in the process of ferroptosis such as lipid peroxidation, glutathione peroxidase 4 enzyme activity, and iron metabolism. Moreover, the association of ferroptosis with the pathophysiology of some neurodegenerative diseases, namely Alzheimer’s, Parkinson’s, and Huntington’s diseases, has also been addressed.

Plasma membrane rupture (PMR) in dying cells undergoing pyroptosis or apoptosis requires the cell-surface protein NINJ1 1 . PMR releases pro-inflammatory cytoplasmic molecules, collectively called damage-associated molecular patterns (DAMPs), that activate immune cells. Therefore, inhibiting NINJ1 and PMR may limit the inflammation that is associated with excessive cell death. Here we describe an anti-NINJ1 monoclonal antibody that specifically targets mouse NINJ1 and blocks oligomerization of NINJ1, preventing PMR. Electron microscopy studies showed that this antibody prevents NINJ1 from forming oligomeric filaments. In mice, inhibition of NINJ1 or Ninj1 deficiency ameliorated hepatocellular PMR induced with TNF plus d -galactosamine, concanavalin A, Jo2 anti-Fas agonist antibody or ischaemia–reperfusion injury. Accordingly, serum levels of lactate dehydrogenase, the liver enzymes alanine aminotransaminase and aspartate aminotransferase, and the DAMPs interleukin 18 and HMGB1 were reduced. Moreover, in the liver ischaemia–reperfusion injury model, there was an attendant reduction in neutrophil infiltration. These data indicate that NINJ1 mediates PMR and inflammation in diseases driven by aberrant hepatocellular death. A monoclonal antibody that binds NINJ1 and inhibits NINJ1 oligomerization prevents plasma membrane rupture in dying cells, resulting in decreased inflammation of surrounding tissue in mice.

Mice with macrophages deficient in fatty acid synthase exhibit lower levels of diabetes-related insulin resistance and inflammation, qualities that are restored on addition of exogenous cholesterol. Cells must make fat to respond to fat Dietary fat promotes chronic inflammation and insulin resistance. This involves the recruitment of macrophages to adipose tissue. This study shows that macrophage fatty acid synthase (FAS) is necessary for diet-induced inflammation. Deleting FAS from macrophages alters membrane order and composition of the macrophage, impairing retention of plasma membrane cholesterol and Rho GTPase trafficking required for cell adhesion, migration and activation. Hence, the absence of FAS prevents adipose macrophage recruitment, chronic inflammation and diet-induced insulin resistance in mice. Dietary fat promotes pathological insulin resistance through chronic inflammation 1 , 2 , 3 . The inactivation of inflammatory proteins produced by macrophages improves diet-induced diabetes 4 , but how nutrient-dense diets induce diabetes is unknown 5 . Membrane lipids affect the innate immune response 6 , which requires domains 7 that influence high-fat-diet-induced chronic inflammation 8 , 9 and alter cell function based on phospholipid composition 10 . Endogenous fatty acid synthesis, mediated by fatty acid synthase (FAS) 11 , affects membrane composition. Here we show that macrophage FAS is indispensable for diet-induced inflammation. Deleting Fasn in macrophages prevents diet-induced insulin resistance, recruitment of macrophages to adipose tissue and chronic inflammation in mice. We found that FAS deficiency alters membrane order and composition, impairing the retention of plasma membrane cholesterol and disrupting Rho GTPase trafficking—a process required for cell adhesion, migration and activation. Expression of a constitutively active Rho GTPase, however, restored inflammatory signalling. Exogenous palmitate was partitioned to different pools from endogenous lipids and did not rescue inflammatory signalling. However, exogenous cholesterol, as well as other planar sterols, did rescue signalling, with cholesterol restoring FAS-induced perturbations in membrane order. Our results show that the production of endogenous fat in macrophages is necessary for the development of exogenous-fat-induced insulin resistance through the creation of a receptive environment at the plasma membrane for the assembly of cholesterol-dependent signalling networks.

High-grade gliomas are lethal brain cancers whose progression is robustly regulated by neuronal activity. Activity-regulated release of growth factors promotes glioma growth, but this alone is insufficient to explain the effect that neuronal activity exerts on glioma progression. Here we show that neuron and glioma interactions include electrochemical communication through bona fide AMPA receptor-dependent neuron–glioma synapses. Neuronal activity also evokes non-synaptic activity-dependent potassium currents that are amplified by gap junction-mediated tumour interconnections, forming an electrically coupled network. Depolarization of glioma membranes assessed by in vivo optogenetics promotes proliferation, whereas pharmacologically or genetically blocking electrochemical signalling inhibits the growth of glioma xenografts and extends mouse survival. Emphasizing the positive feedback mechanisms by which gliomas increase neuronal excitability and thus activity-regulated glioma growth, human intraoperative electrocorticography demonstrates increased cortical excitability in the glioma-infiltrated brain. Together, these findings indicate that synaptic and electrical integration into neural circuits promotes glioma progression. Neurons form synapses onto glioma cells, and depolarization of glioma membranes promotes glioma growth in vivo, whereas blocking electrochemical signalling blocks tumour growth.

Ferroptosis is a non-apoptotic form of cell death that can be triggered by small molecules or conditions that inhibit glutathione biosynthesis or the glutathione-dependent antioxidant enzyme glutathione peroxidase 4 (GPX4). This lethal process is defined by the iron-dependent accumulation of lipid reactive oxygen species and depletion of plasma membrane polyunsaturated fatty acids. Cancer cells with high level RAS-RAF-MEK pathway activity or p53 expression may be sensitized to this process. Conversely, a number of small molecule inhibitors of ferroptosis have been identified, including ferrostatin-1 and liproxstatin-1, which can block pathological cell death events in brain, kidney and other tissues. Recent work has identified a number of genes required for ferroptosis, including those involved in lipid and amino acid metabolism. Outstanding questions include the relationship between ferroptosis and other forms of cell death, and whether activation or inhibition of ferroptosis can be exploited to achieve desirable therapeutic ends.

Brain tumor patients commonly present with epileptic seizures. We show that tumor-associated seizures are the consequence of impaired GABAergic inhibition due to an overall loss of peritumoral fast spiking interneurons (FSNs) concomitant with a significantly reduced firing rate of those that remain. The reduced firing is due to the degradation of perineuronal nets (PNNs) that surround FSNs. We show that PNNs decrease specific membrane capacitance of FSNs permitting them to fire action potentials at supra-physiological frequencies. Tumor-released proteolytic enzymes degrade PNNs, resulting in increased membrane capacitance, reduced firing, and hence decreased GABA release. These studies uncovered a hitherto unknown role of PNNs as an electrostatic insulator that reduces specific membrane capacitance, functionally akin to myelin sheaths around axons, thereby permitting FSNs to exceed physiological firing rates. Disruption of PNNs may similarly account for excitation-inhibition imbalances in other forms of epilepsy and PNN protection through proteolytic inhibition may provide therapeutic benefits. Brain tumours are associated with epilepsy. Here the authors show, using a mouse model, that the degradation of perineuronal nets around fast spiking interneurons near the tumour contribute to seizures by increasing their membrane capacitance and firing.

Key Points Although for a long time necrosis was considered to be a purely accidental cell death subroutine, multiple lines of evidence now show that necrotic cell death can be regulated, both in its occurrence and in its course. The term 'necroptosis' was introduced by Yuan's research group in 2005 to indicate 'programmed' (as opposed to 'accidental') necrosis. The best characterized signal transduction cascade leading to necroptosis is initiated by ligand-bound tumour necrosis factor (TNF) receptor 1 (TNFR1), which allows for the assembly of a cytoplasmic supramolecular complex — TNFR complex I — that includes (among other proteins) TNFR-associated death domain (TRADD), cellular inhibitor of apoptosis 1 (cIAP1), cIAP2 and receptor-interacting protein kinase 1 (RIP1; also known as RIPK1). In complex I, RIP1 can be ubiquitylated by cIAPs and deubiquitylated by cylindromatosis (CYLD) and A20 (also known as TNFAIP3). Whereas ubiquitylated RIP1 promotes the activation of the nuclear factor κB (NF-κB) system, deubiquitylated RIP1 functions as a cell death-inducing kinase. On TNFR1 internalization, the so-called TNFR complex II is formed, which contains TRADD, FAS-associated protein with a death domain (FADD) and caspase 8. Normally, caspase 8 becomes activated in complex II, thereby igniting a pro-apoptotic caspase cascade. When caspase activation is prevented, however, RIP1 physically and functionally interacts with RIP3 (also known as RIPK3), thereby generating a necroptosis-inducing complex known as the necrosome. Necroptosis can also be ignited by pathogen recognition receptors, including Toll-like receptors, NOD-like receptors and retinoic acid-inducible gene I-like receptors, as well as in response to DNA damage, presumably by a poly(ADP-ribose) polymerase-1 (PARP1)-dependent signalling pathway. Although the underlying molecular mechanisms remain obscure, reactive oxygen species (ROS), bioenergetic metabolic cascades and the release of cytotoxic factors from lysosomes and mitochondria all contribute to the execution of necroptosis. Regulated necrosis has been seen in multiple, evolutionarily distant model organisms, including yeast, nematodes, fruit flies, rodents, primates and human cells, corroborating the notion that necroptosis may represent a phylogenetically conserved mechanism for programmed cell death. Numerous in vivo studies indicate that the inhibition of necroptosis (by genetic means or by RIP1-targeting agents called necrostatins) can confer consistent cytoprotection, suggesting that necroptosis may constitute a promising target for drug development. Although necrosis was regarded as an uncontrolled mode of cell death, evidence now shows that it can be highly regulated. The initiation of programmed necrosis (necroptosis) by death receptors requires receptor-interacting protein 1 (RIP1) and RIP3, and its execution involves the active disintegration of mitochondrial, lysosomal and plasma membranes. For a long time, apoptosis was considered the sole form of programmed cell death during development, homeostasis and disease, whereas necrosis was regarded as an unregulated and uncontrollable process. Evidence now reveals that necrosis can also occur in a regulated manner. The initiation of programmed necrosis, 'necroptosis', by death receptors (such as tumour necrosis factor receptor 1) requires the kinase activity of receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3), and its execution involves the active disintegration of mitochondrial, lysosomal and plasma membranes. Necroptosis participates in the pathogenesis of diseases, including ischaemic injury, neurodegeneration and viral infection, thereby representing an attractive target for the avoidance of unwarranted cell death.

Cell membranes with their selective permeability play important functions in the tight control of molecular exchanges between the cytosol and the extracellular environment as the intracellular membranes do within the internal compartments. For this reason the plasma membranes often represent a challenging obstacle to the intracellular delivery of many anti-cancer molecules. The active transport of drugs through such barrier often requires specific carriers able to cross the lipid bilayer. Cell penetrating peptides (CPPs) are generally 5–30 amino acids long which, for their ability to cross cell membranes, are widely used to deliver proteins, plasmid DNA, RNA, oligonucleotides, liposomes and anti-cancer drugs inside the cells. In this review, we describe the several types of CPPs, the chemical modifications to improve their cellular uptake, the different mechanisms to cross cell membranes and their biological properties upon conjugation with specific molecules. Special emphasis has been given to those with promising application in cancer therapy.